1. Field of the Invention
This invention relates, in general, to incandescent lamps. In particular, the invention relates to an incandescent lamp having an optical interference film formed on the surface thereof to increase the color temperature of light radiated therefrom.
2. Discussion of the Background
In recent years, a light source, a so-called cool beam, having a small amount of infrared light and also having good color rendering properties has been desired as a spot light in stores, etc. To achieve this light source in an incandescent lamp, conventionally a color temperature conversion filter has been used in the lamp as a front glass or a dichroic mirror is used to cut the unnecessary light element from the spectrum. However, in such light sources using the color temperature conversion filter or the dichroic mirror, the efficiency of the lamp is lowered because of the elimination of the unnecessary light elements. Thus, a high color temperature of light from the lamp is not achieved.
An interference filter including a plurality of dielectric films of TiO.sub.2 --SiO.sub.2 has a high temperature resistance and may be formed on either inner or outer surface of the bulb of the incandescent lamp, as an infrared reflecting film, to reflect a large amount of infrared light. The filament disposed in the bulb is heated by reflected infrared light to improve the efficiency of the lamp. The above-described interference filter may be applied to an incandescent lamp, e.g., a halogen lamp, whose bulb surface temperature is relatively high. Thus, the incandescent lamp on which the conventional interference filter is formed has a high color temperature, as compared with the lamp on which the color temperature conversion filter or the dichroic mirror is formed. However, it is difficult to achieve desirable color rendering properties similar to that of sunlight even if an interference filter of a .lambda./4 film normally used as an infrared reflecting film or an interference filter of a .lambda./4 film wherein a .lambda./2 low refractive index layer is sandwiched between high refractive index layers is applied to the lamp. This is because the transmttance of these conventional interference filters rapidly decreases or undulates in the range between 400 nm and 700 nm, and the spectrum of light output from the lamp through the conventional filter is different from that of sunlight. Thus, the lamp on which this conventional interference filter is formed can not achieve good color rendering properties at a high color temperature, e.g., greater than about 3400K.